The seal of the Senate, based on the Great Seal of the United States, includes a scroll inscribed with E Pluribus Unum floating across a shield with thirteen stars on top and thirteen vertical stripes on the bottom. Olive and oak branches symbolizing peace and strength grace the sides of the shield, and a red liberty cap and crossed fasces represent freedom and authority. Blue beams of light emanate from the shield. Surrounding the seal is the legend, "United States Senate." The seal is affixed to impeachment documents and resolutions of consent to international treaties. It also appears on presentation copies of Senate resolutions recognizing appointments, commendations, and notable achievements.

This current seal represents the third design since 1789. The first seal showed an eagle with a shield on its breast, olive branches in its left talon, and arrows in its right. Above the eagle were rays of light emanating from clouds, representing the emergence of the new nation. Encircling the design was the legend "Senate of the United States." The first known use of this seal was on the March 1798 impeachment summons of Tennessee Senator William Blount. The seal authenticated the summons and asserted the right of the Senate to try Blount. Six years later, the seal appeared on another impeachment summons, this time for Federal Judge John Pickering.

By 1830, the first Senate seal was either lost or unserviceable. A new seal was commissioned from Robert Lanphier, Jr., a Washington D.C. engraver and jeweler. This second design was inspired by Greek and Roman models, depicting three female figures that symbolized freedom, justice, and power. An eagle perched atop the figures, and twenty-four links of a chain bordering the seal represented the twenty-four states then in the Union. During the impeachment trial of President Andrew Johnson, the seal authenticated both the presidential summons and copies of documents submitted in evidence. The second seal was used until 1880.

When the nation celebrated its one hundred years of independence in 1876, a new Great Seal of the United States was created and put into use in 1885, prompting the Senate to revise its own seal. The old seal had been discovered in the Capitol basement in 1880, worn down from age and perhaps last used in an impeachment trial in 1876. Louis Dreka, an engraver and stationer from Philadelphia, received $35 to produce a new seal, measuring one-and-a-half inches in diameter. The 1885 design is still in use today.

The seal is kept in the custody of the Secretary of the Senate, in accordance with a resolution adopted in 1886 which mandates that it be used to authenticate transcripts, copies, and certificates as directed by the Senate. In the twentieth century, the Secretary of the Senate has authorized official use of the seal by the majority and minority leaders.